The present invention relates to a power transfer system for a four-wheel drive motor vehicle and, in particular, to an adapter for converting a single offset transfer case into a one and one half offset transfer case assembly.
In view of the increased popularity of four-wheel drive vehicles, a plethora of power transfer systems are currently utilized for distributing tractive power (i.e., drive torque) to all four wheels of the motor vehicle. Conventionally, most four-wheel drive power transfer systems include a transfer case having an input shaft driven by the transmission output shaft, a rear output shaft driven by the input shaft and which is interconnected via a rear prop shaft to a rear axle assembly for driving the rear wheels, a front output shaft interconnected via a front prop shaft to a front axle assembly for driving the front wheels, and a torque transfer arrangement for continuously or selectively transferring drive torque from the rear output shaft to the front output shaft.
Currently, several different types of power transfer systems are being utilized in four-wheel drive vehicles. In "part-time" four-wheel drive systems, the transfer case is equipped with a mode shift mechanism which permits the vehicle operator to selectively couple the front output shaft to the rear output shaft for shifting the vehicle from a two-wheel drive mode into a four-wheel drive mode. An example of a part-time transfer case is shown and disclosed in commonly-owned U.S. Pat. No. 5,159,847 to Williams et al. Alternatively, "on-demand" power transfer systems are used to automatically transfer drive torque to the non-driven wheels when traction is lost at the driven wheels without any input or action required by the vehicle operator. For instance, a speed-sensitive torque transfer device can be installed in the transfer case for transferring drive torque from the rear output shaft to the front output shaft when an excessive speed differential occurs therebetween. Commonly-owned U.S. Pat. No. 5,358,454 to Bowen et al. disclosed a geared traction unit suitable for such an application. An example of an electronically-controlled on-demand power transfer system is disclosed in commonly-owned U.S. Pat. No. 5,363,938 to Wilson et al. Finally, in "full-time" four-wheel drive systems, the transfer case is equipped with an interaxle differential for continuously distributing the drive torque between the front and rear output shafts while permitting speed differentiation therebetween. Commonly-owned U.S. Pat. No. 4,677,873 to Eastman et al. discloses an exemplary full-time transfer case. Many full-time transfer cases are also equipped with a "slip limiting" device for selectively or automatically limiting or preventing excessive speed differentiation across the interaxle differential as a result of excessive wheel slip. Commonly-owned U.S. Pat. Nos. 5,078,660 and 5,106,351 to Williams et al. disclose exemplary full-time transfer cases equipped with a viscous coupling for limiting slip across the interaxle differential.
The above-noted types of transfer cases are commonly designed to have one or both output shafts offset from the input shaft. In the majority of transfer cases, the rotational axis of the front output shaft is offset from the common rotational axis shared by the input shaft and the rear output shaft. This arrangement is commonly known as a single-offset transfer case. For example, FIG. 1 illustrates an exemplary power transfer system 10 for a four-wheel drive vehicle. Power transfer system 10 includes a front driveline 12 and a rear driveline 14 both of which are drivable from a source of power, such as engine 16 and transmission 18. Transmission 18 includes an output shaft 20 supported for rotation about a rotary axis "O". A transfer case 22 is provided for transmitting drive torque from transmission output shaft 20 to front driveline 12 and rear driveline 14. Specifically, an input shaft 24 of transfer case 22 is coupled to transmission output shaft 20 for rotation about a rotary axis "I" which is commonly aligned with axis "O". Front driveline 12 includes a pair of front wheels 26 connected at opposite ends of a front axle assembly 28 which is coupled by a differential 30 to one end of a front prop shaft 32, the opposite end of which is coupled to a front output shaft 34 of transfer case 22. As seen, front output shaft 34 is supported in transfer case 22 for rotation about a rotary axis "F". Rear driveline 14 includes a pair of rear wheels 36 connected at opposite ends to a rear axle assembly 38 which is coupled by a differential 40 to one end of a rear prop shaft 42, the opposite end of which is coupled to a rear output shaft 44 of transfer case. Rear output shaft 44 is supported in transfer case 22 for rotation about a rotary axis "R" which is commonly aligned with rotary axis "I" of input shaft 24. Thus, transfer case 22 is of the single-offset type.
A smaller number of transfer cases have the front and rear output shafts commonly aligned for rotation about a rotary axis that is offset from the rotary axis of the input shaft. This arrangement is commonly known as a double-offset transfer case. While single and double offset transfer cases are used in the majority of conventional four-wheel drive arrangements, the need exists to provide alternatives for use in those vehicular applications in which such conventional transfer cases are not applicable. Unfortunately, the substantial cost and leadtime required to develop an alternative transfer case for such a special vehicular application is not typically justified.